The present invention relates to an exhaust emission control system for an internal combustion engine, and more particularly to an exhaust emission control system including a nitrogen oxide removing device for removing nitrogen oxides and for determining deterioration of the nitrogen oxide removing device.
In the case of setting the air-fuel ratio of an air-fuel mixture to be supplied to an internal combustion engine in a lean region with respect to a stoichiometric ratio (i.e., in the case of carrying out a so-called lean operation), the emission amount of nitrogen oxides (which will be hereinafter referred to as xe2x80x9cNOxxe2x80x9d) tends to be increased. To cope with this, a known technique for exhaust emission control includes providing an NOx removing device containing an NOx absorbent for absorbing NOx in the exhaust system of the engine. The NOx absorbent has a characteristic that in the condition where the air-fuel ratio is set in a lean region with respect to the stoichiometric ratio and the oxygen concentration in exhaust gases is therefore relatively high (the amount of NOx is large) (this condition will be hereinafter referred to as xe2x80x9cexhaust lean conditionxe2x80x9d), the NOx absorbent absorbs NOx, whereas in the condition where the air-fuel ratio is set in a rich region with respect to the stoichiometric ratio and the oxygen concentration in exhaust gases is therefore relatively low (this condition will be hereinafter referred to as xe2x80x9cexhaust rich conditionxe2x80x9d), the NOx absorbent discharges the absorbed NOx. The NOx removing device containing this NOx absorbent is configured so that NOx discharged from the NOx absorbent in the exhaust rich condition is reduced by HC and CO and then exhausted as nitrogen gas, while HC and CO are oxidized and then exhausted as water vapor and carbon dioxide.
As emission control means, a three-way catalyst having oxidizing and reducing functions is also widely used, and an exhaust emission control system including such a three-way catalyst in addition to an NOx removing device is known in the art (e.g., Japanese Patent Laid-open No. Hei 11-93744). The three-way catalyst is arranged upstream of the NOx removing device in the exhaust system. This exhaust emission control system further includes two air-fuel ratio sensors respectively arranged upstream and downstream of the NOx removing device. In this exhaust emission control system, the deterioration of the NOx removing device is determined according to outputs from the two air-fuel ratio sensors when changing the air-fuel ratio from a lean air-fuel ratio to a rich air-fuel ratio or vice versa.
More specifically, when changing the air-fuel ratio from a lean air-fuel ratio to a rich air-fuel ratio or vice versa, the time period during which the output from each of the air-fuel ratio sensors arranged upstream and downstream of the NOx removing device retains a value corresponding to the stoichiometric ratio is measured, and the deterioration is then determined according to the time period measured above.
However, in the above conventional control system, variations in response characteristics of the air-fuel ratio sensors are not considered, so that the results of measurement change according to the response characteristics, causing a problem that the determination of deterioration becomes inaccurate in some cases. More specifically, in the above conventional control system, a time period RCU during which the output from the air-fuel ratio sensor provided upstream of the NOx removing device (downstream of the three-way catalyst) retains a value corresponding to the stoichiometric ratio and a time period RCD during which the output from the air-fuel ratio sensor provided downstream of the NOx removing device retains a value corresponding to the stoichiometric ratio are measured, and the deterioration of the NOx removing device is determined by a time difference TSTR=RCDxe2x88x92RCU. This time difference TSTR takes different values, for example, between in the case that the upstream air-fuel ratio sensor has a quick-response characteristic and the downstream air-fuel ratio sensor has a slow-response characteristic and in the case that the former has a slow-response characteristic and the latter has a quick-response characteristic. As a result, the accuracy of the determination of deterioration is reduced.
It is accordingly an object of the present invention to provide an exhaust emission control system which can determine the degree of deterioration of an NOx removing device more accurately than the prior art in the case of arranging sensors upstream and downstream of the NOx removing device.
The present invention provides an exhaust emission control system for an internal combustion engine, having nitrogen oxide removing means provided in an exhaust system of the internal combustion engine for absorbing nitrogen oxides contained in exhaust gases in an exhaust lean condition and reducing the absorbed nitrogen oxides in an exhaust rich condition. This control system is comprises a first oxygen concentration sensor provided upstream of the nitrogen oxide removing means for detecting the oxygen concentration in the exhaust gases; a second oxygen concentration sensor provided downstream of the nitrogen oxide removing means for detecting the oxygen concentration in the exhaust gases; first measuring means for measuring a first response delay time period from the time when the output value from the first oxygen concentration sensor has changed to a value indicative of a rich air-fuel ratio to the time when an output value from the second oxygen concentration sensor becomes a value indicative of a rich air-fuel ratio, after changing the air-fuel ratio of the air-fuel mixture to be supplied to the engine from a lean region to a rich region with respect to a stoichiometric ratio; second measuring means for measuring a second response delay time period from the time when the output value from the first oxygen concentration sensor has changed to a value indicative of a lean air-fuel ratio, to the time when the output value from the second oxygen concentration sensor becomes a value indicative of a lean air-fuel ratio, after changing the air-fuel ratio from the rich region to the lean region with respect to the stoichiometric ratio; and deterioration determining means for determining deterioration of the nitrogen oxide removing means according to the first and second response delay time periods.
With this arrangement, the first response delay time period from the time when the output value from the first oxygen concentration sensor has been changed to a value indicative of a rich air-fuel ratio to the time when the output value from the second oxygen concentration sensor becomes a value indicative of a rich air-fuel ratio is measured after changing the air-fuel ratio of an air-fuel mixture to be supplied to the engine from a lean region to a rich region with respect to the stoichiometric ratio. Further, the second response delay time period from the time when the output value from the first oxygen concentration sensor has been changed to a value indicative of a lean air-fuel ratio to the time when the output value from the second oxygen concentration sensor becomes a value indicative of a lean air-fuel ratio is measured after changing the air-fuel ratio from the rich region to the lean region with respect to the stoichiometric ratio. Then, the deterioration of the nitrogen oxide removing means is determined according to the first and second response delay time periods measured above. It is experimentally confirmed that the second response delay time period does not largely depend on whether or not the nitrogen oxide removing means is deteriorated, but reflects variations in characteristics of the oxygen concentration sensors. Accordingly, by using the second response delay time period with the first response delay time period reflecting the degree of deterioration of the nitrogen oxide removing means, the influence of variations in characteristics of the oxygen concentration sensors can be eliminated to allow more accurate determination of deterioration.
Preferably, the deterioration determining means includes correcting means for correcting the first response delay time period according to the second response delay time period, and determines the deterioration according to the first response delay time period corrected by the correcting means.
Preferably, the correcting means calculates a correction coefficient according to a change in the operating condition of the engine during the period from the time of measurement of the first response delay time period to the time of measurement of the second response delay time period, corrects the second response delay time period by using the correction coefficient, and corrects the first response delay time period by using the corrected second response delay time period.
Preferably, the exhaust emission control system further includes a three-way catalyst provided upstream of the first oxygen concentration sensor, and three-way catalyst deterioration determining means for determining deterioration of the three-way catalyst, wherein the deterioration determining means determines the deterioration of the nitrogen oxide removing means according to the first and second response delay time periods and the degree of deterioration of the three-way catalyst. In this case, the exhaust emission control system further includes a third oxygen concentration sensor provided upstream of the three-way catalyst, wherein the three-way catalyst deterioration determining means determines the degree of deterioration of the three-way catalyst according to an output value from the third oxygen concentration sensor and an output value from the first oxygen concentration sensor.
Preferably, the deterioration determining means calculates a first average value from a plurality of measured values of the first response delay time period and a second average value from a plurality of measured values of the second response delay time period, and determines the deterioration of the nitrogen oxide removing means according to the first and second average values.
Other objects and features of the invention will be more fully understood from the following detailed description and appended claims when taken with the accompanying drawings.